Method of and appartus for producing a compression product

ABSTRACT

A method of producing components by compressing powdered material in a hollow die includes the use of a sleeve in the die to reduce the pressure required to remove the product from the die after compression, and to reduce cracking in the product after spring-back upon release of the component. An elastically deformable sleeve having a cylindrical internal surface and conical external surface is inserted into a tapered aperture in a die plate so as to reduce elastically the internal diameter of the sleeve. The component is produced by compressing powdered material in the interior of the sleeve by upper and lower punches. The upper punch is then removed and the sleeve is partially removed with the product from the die, by movement against the direction of the taper. The interior dimension of the sleeve increases elastically and the product is then removed from the sleeve by raising the lower punch.

This application claims benefit of international applicationPCT/GB94/01941, filed Sep. 7, 1994.

The present invention relates to a method of, and apparatus for,producing a product by compression of material, particularly but notexclusively by compression of powdered metallic material.

Many components for industrial applications are manufactured by thepowder metallurgy route in which metal powders are formed into thedesired shape with very little waste material and with high dimensionalaccuracy. However, the mechanical and physical properties of powdermetallurgical materials depend significantly on the final density of thecomponent. In general, mechanical strength improves dramatically asdensity is increased and, for example, in magnetic materials thepermeability increases with increase in density.

Many shaped components are made by pressing powder in fixed dies using,in its simplest form, a die with top and bottom punches. FIGS. 1a, b andc show a section through a die set used to produce a cylindrical object.Powder 11 is placed in the die 12 and the top and bottom punches 13 and14 compress the powder (FIG. 1a). The top punch 13 is then removed as inFIG. 1b, and the compressed powder is ejected as a compact 15 which hassufficient strength to be handled, but insufficient to be used as acomponent (FIG. 1c). The compact is subsequently passed through afurnace at the appropriate temperature to induce diffusion between thepowder particles. This so-called sintering process converts the pressedpowder into a continuous material with sufficient strength for theproposed application of the component.

One of the problems associated with pressing in a fixed die systemrelates to the compressibility of the powder and how the pressed compactis removed from the die. As the powder is pressed in the die set itsdensity increases due to the increasing pressing pressure used duringthe pressing cycle. The compacted powder exerts an internal pressure Pion the die walls, and the greater the axially applied pressure Pa thegreater the internal pressure on the die walls (as shown in FIG. 1a).When the pressing pressure is removed there is still a residual stressin the compact 15 which exerts a pressure Pir on the die walls (as shownin FIG. 1b). It is necessary to use an ejection force Pe (FIG. 1b) toovercome this die wall pressure and to eject the compact from the die.

As the compact 15 is being ejected from the die 12 (as shown in FIG. 2),it is subjected to external shear forces Fe on its external surface dueto the restraining effect of the die. As the axial pressing pressure Pa(previously applied) increases, this shear force Fe (arising duringrelease) also increases, making it more difficult to eject thecomponent. Hence the ejection force Pe which is required increases, andthere is a danger of the compact damaging the die walls, and the compactitself being damaged by the contact with the die walls. Additionally, asthe compact 15 emerges from the die 12 it is no longer restrained by thedie as in FIG. 3, and as there are elastic stresses in the compact dueto the pressing operation the compact is able to change its shape torelieve these elastic stresses. This is known as `spring-back`. Thismeans, for example as shown in FIG. 3, that a powder compact 15compressed in a die 12 of a specific internal diameter D, will, onejection, have a diameter D+dc, where dc is the increase in diameter ofthe compact on ejection. This spring-back effect can result in thepressed compact developing cracks at right angles to the pressingdirection, due to the difference in diameter of the part of the compactstill in the die (with a diameter D), and the part which has beenejected free from the die (with a diameter D+dc). At the change indiameter at the top face of the die cracks can be formed.

In order to produce high density components it is necessary to obtain ashigh a density in the pressed powder compact as possible, but as is nowevident the higher the pressing pressure the more difficult it is toremove the compact from the die. The ejection forces will need to behigher the higher the compaction pressure, and there is a highprobability that the die, and/or the compact will be damaged during theejection stroke. There is also the problem that the spring-back effectwill increase as the pressing pressure increases leading to damagedcompacts on ejection. These problems normally mean that pressingpressures, and therefore pressed densities, are restricted when usingfixed dies.

In SU-A-1315135 (Zlobin et al) there is disclosed apparatus forproducing a compacted article by compression of metal powder. A die isformed of three partible units and has a slotted elastic shell insidethe die. The external surface of the die units is conical and isenclosed in a corresponding conical hole of a thrust ring. Axialmovement of the thrust ring clamps the units of the die towards eachother and compresses the slotted shell closing its slot. Then, metalpowder is charged into the shell acting as an inner liner of the die,and the powder is compacted by a single ended pressing punch. Aftercompaction, the thrust ring is pressed downwardly which opens the die,releasing the compacted article.

One disadvantage of such an arrangement in practical use is that thepowder to be compacted finds its way into the slot in the inner liningof the die. The behaviour of metallic powders during compaction is notthat of a fluid. The region of the powder adjacent the moving punch willcompact first, whilst powder remote from the piston will not at first becompressed. The pressure of compaction will mean that some opening atthe slot will exist, and powder will be forced into this opening. Alsogenerally during operation of the machine, despite cleaning, powder willbuild up in the crack of the shell, and that powder will itself becompacted during subsequent operations. The result is that it will notbe possible to close completely the slot in the shell in subsequentoperations, which means that accuracy is lost both in the dimensions ofthe compact produced, and in the provision of protrusions at the surfaceof the compact where the slot has been positioned. Other disadvantagesof the arrangement are that the thrust ring will not apply pressureevenly to the inner shell. Furthermore, since the shell is slotted, thetensions in the shell will vary from compression where the edges of aslot abut each other, to tension on the outside of the shelldiametrically opposite the slot. Such tensions will prevent compressionof the slotted shell uniformly around the perimeter of the lining, whichwill again reduce accuracy of the compact being produced.

In a paper entitled "NOVEL METHODS OF POWDER COMPACTION" by G. I.Begenkoff and G. B. Zlobin, at pages 289 to 292 of the report of PM90,World Conference on Powder Metallurgy held on 2-6 Jul., 1990 in London,there is disclosed apparatus for producing a compact from powder. Theapparatus comprises a die formed of segments and having an outer conicalsurface, the segments being held together by a holder having a taperedopening corresponding to the taper of the die segments and in which thedie segments are positioned. A single ended, upper pressing ramcompresses powder in the segmented die against inwardly directed flangesof the die segments, and at the same time presses the die segments intothe conical opening in The holder. During formation of the compact, thecompacting pressure is transferred through the compact to the lower ramand die segment flanges, which causes the die segments to be radiallycompressed by the conical side walls of the tapered holder. When theload is removed from the upper ram, the die segments move apart underthe lateral pressure exerted by the holder, sliding upwards along theinclined faces of the holder. The value of the taper angle of the holderis more than the angle of friction between the die segment and theholder.

The disadvantages of this arrangement are similar to those in the patentmentioned hereinbefore, in that the separate die segments will againprovide openings between the segments into which powder will find itsway during compression. This is particularly acute in the example of thereferenced paper, because the force provided to compress inwardly thedie segments arises from the compaction force of the ram compressing theproduct material. Thus at the beginning of She compression stroke, thedie segments will not be compressed inwardly, leaving even larger gapsfor the powder to migrate into. Other disadvantages include the factthat it is not possible to vary the inward force on the die segmentsindependently of the load force applied for compaction of the product.

According to the present invention there is provided a method ofproducing a product by compression of material, comprising the steps of:providing in a hollow die a compressed lining which is elasticallycompressed so as to reduce the internal size of the lining relative tothe internal size before compression, compressing product material inthe lining to produce a compressed product, releasing the lining atleast partially from the die to produce an increase in the internal sizeof the lining, and removing the compressed product from the lining, inwhich the lining is continuous around the interior the die.

The provision of a continuous lining avoids the difficulty of powderfinding its way into slots or other openings with consequent lack ofaccuracy. It is preferred that the lining is compressed by elasticdeformation of the lining material uniformly around the perimeter of thelining. The required reduction in internal size of the lining can beobtained not by the movement of separated parts of a lining or dietowards each other but by the uniform elastic deformation of the bulkmaterial of the lining, as a result of inward pressure applied to thelining. This allows accuracy to be maintained, even in die shapes havinga complicated interior surface, by arranging for uniform forces to beapplied around the lining, to produce a smooth continuous elasticdeformation of the lining material. This produces an internal shape ofthe lining which is reduced in size, but maintains dimensional integritywith the desired shape for the finally compressed product.

It is believed that in the prior art set out above a slot in a sleevewas used because it was thought that a large recovery movement wasrequired upon release of the sleeve and that this could only be achievedby a slot. It has now been found, unexpectedly, that the elasticrecovery from a continuous lining can be made to be of the same order asthe springback of the component being made, so that a continuous liningcan be used, which gives rise to an industrially viable process.

It is to be appreciated that the steps set out in accordance with theinvention are not necessarily performed separately in the order given,and that the order may be varied, and indeed may overlap. For examplethe reduction in size of the lining may be produced partly or completelybefore insertion of the lining in the die, for example by compression ofthe lining in another member before insertion into the die. In otherarrangements, the internal size of the lining may be decreased duringthe pressing operation itself. However, preferably the compression ofthe lining in the die is achieved during the step of inserting thelining into the die. The material to be compressed may be placed in thelining before or after the lining is inserted into the die, but normallythe material will be inserted after the lining is inserted into the die.The invention is particularly applicable where the method includesplacing the product material in the interior of the lining in powderedform, and compressing the material into a rigid product.

Depending upon the shape and application of the lining, the changes ininternal and/or external size of the lining may be changes in one ormore than one dimension. Although the lining may assume a number ofshapes, depending upon the shape of the die, the invention isparticularly applicable where the lining is a sleeve and the methodincludes inserting the sleeve into the die along the direction of acommon axis of the sleeve and the die. Preferably the exterior of thesleeve and the interior of the die are both tapered, and the methodincludes inserting the sleeve into the die in the direction in which thesleeve and die are tapered.

The invention has particular application where the lining is compressedby the step of compressing the lining by elastic deformation of thelining material uniformly around the perimeter of the lining, andpreferably is compressed by the step of compressing the lining beforethe step of compressing the product material to produce the compressedproduct. In some preferred forms, the method includes the step ofproducing an adjustable, selectable compression of the lining, wherebythe increase in internal size of the lining on release of the liningfrom the die can be selected in relation to the expected increase inexternal size of the product on release from the die. However, in someproduction examples, the apparatus used will be set so as to produce apredetermined compression of the lining, for a particular product to bemade.

The amount of compression of the lining will be chosen according to therequirements of the product, but preferably the method includescompressing the lining to an extent such that the increase in internalsize of the lining on release of the lining from the die is in therange + or -20% of the increase in external size of the product onrelease from the die, preferably the range being + or -10%. Normally thelining will be compressed to an extent such that the increase ininternal size of the lining during release from the die is at leastequal to the expansion of the product after release from the die,preferably substantially equal to the expansion of the product.

Although it may be arranged that the product has a generally circularperimeter, and the said increase of size of the product and the liningis an increase in radius thereof, other shapes of die and lining may beprovided, such as an oval, or a complex shape such as that of an engineconnecting rod. The outer surface of the sleeve and the inner surface ofthe die may assume a number of shapes, but conveniently the outersurface of the sleeve and the inner surface of the die are both circularin cross section.

In many arrangements the interior surface of the sleeve is circular incross section. However the interior surface of the sleeve may have theconfiguration of a mould for producing an article of generally circularcross section but having a varying shape around its perimeter, e.g. theconfiguration of a mould for producing a gear wheel. Thus the interiorsurface of the sleeve may have a configuration such that the distance ofthe surface from the axis of the sleeve varies around the interiorsurface of the sleeve. In some arrangements the interior surface of thesleeve has a cross section which is constant along the direction of theaxis of the sleeve, but in other arrangements the interior surface ofthe sleeve has a cross section which varies in the direction of the axisof the sleeve, for example in discontinuous steps.

The invention finds particularly preferred application where the liningis a sleeve and the method includes inserting the sleeve into the diealong the direction of a common axis of the sleeve and the die, and inwhich the interior of the die is tapered in the direction of the commonaxis so that insertion of the sleeve produces compression of the sleeveby the die. Preferably the exterior of the sleeve is also tapered, inthe same sense as the taper of the interior of the die, and preferablythe angle of taper of the sleeve is the same as the angle of taper ofthe die. Preferably the angle of taper of the die is in the range 0.5 to10', most preferably in the range 1 to 5°, and particularly preferablyabout 2°.

The invention finds particular application where the hollow die isprovided by an aperture in a die and the method includes compressing thematerial by moving upper and lower punches into the aperture in the diein the interior of the lining. However the invention is equallyapplicable with rotary compaction to densify powder. Rotary compactionis a known process having the following main steps.

The bottom of the top punch of a rotary compaction die set has a conicalsurface and the central axis of the top punch is offset with respect tothe central axis of the die at such an angle that when the top punch islowered onto the powder, a line contact is produced between the toppunch and the powder. This contrasts with the whole of the bottomsurface of the top punch in a conventional die set contacting thesurface of the powder. The line contact in rotary compaction is made torotate about the centre line of the die by a suitable mechanical means.Methods to produce this are well known. Nominal line contact means thatmuch higher specific pressures are applied to the powder, resulting inhigh density compacted material.

In accordance with one particular feature of the invention, there isprovided a method of calibrating the die and lining by the steps of:

(a) pressing the sleeve into the die and measuring the change in innerdiameter of the sleeve as a function of the change in axial position ofthe sleeve;

(b) for any particular product material, measuring the compressibilityand spring back as a function of the pressing pressure;

(c) for a required pressing density during production of a compressedproduct, determining from the information of step (b) the spring backwhich would occur in a conventional die; and

(d) determining from the data acquired in step (a) the extent ofinsertion of the sleeve that is required to give a value of decrease ofinner diameter of the sleeve which is equal to the expected spring backdetermined in step (c), or falls within a predetermined range ofdeviation from that springback.

There will now be set out a number of independent aspects of theinvention, which may be utilised independently of the main features setout above. In one further aspect of the invention there may be provideda method of producing a product by compression of material, comprisingthe steps of: providing in a hollow die a compressed sleeve which iselastically compressed so as to reduce the internal size of the sleeverelative to the internal size before compression, inserting into thecompressed sleeve a material to be compressed, compressing the materialin the sleeve to produce a compressed product, releasing the sleeve atleast partially from the die to produce an increase in the internal sizeof the sleeve, and removing the compressed product from the sleeve, inwhich the interior surface of the die and the exterior surface of thesleeve are both tapered, the method including the step of inserting thetapered sleeve into the tapered die and compressing the sleeve by theeffect of the tapered surfaces, before the compression of the materialin the sleeve to produce the product.

In yet another aspect of the invention there may be provided a method ofproducing a product by compression of material, comprising the steps of:providing in a hollow die a compressed lining which is elasticallycompressed so as to reduce the internal size of the lining relative tothe internal size before compression, compressing product material inthe lining to produce a compressed product, releasing the lining atleast partially from the die to produce an increase in the internal sizeof the lining, and removing the compressed product from the lining,including the step of compressing the lining by elastic deformation ofthe lining material uniformly around the perimeter of the lining.

In a yet further aspect, there may be provided in accordance with theinvention a method of producing a product by compression of material,comprising the steps of: providing in a hollow die a compressed liningwhich is elastically compressed so as to reduce the internal size of thelining relative to the internal size before compression, compressingproduct material in the lining to produce a compressed product,releasing the lining at least partially from the die to produce anincrease in the internal size of the lining, and removing the compressedproduct from the lining, including the step of producing an adjustable,selectable, compression of the lining, whereby the increase in internalsize of the lining on release of the lining from the die can be selectedin relation to the expected increase in external size of the product onrelease from the die.

Finally, in accordance with another aspect of the invention, there maybe provided a method of producing a product by compression of material,comprising inserting into a hollow die a core which is elasticallyexpanded; compressing product material in the die around the core; afterthe compression of the product material, reducing the size of the core;and removing the compressed product from the die and from the core.

It is to be appreciated that where features of the invention have beenset out in accordance with a method of the invention, these features mayalso be provided in accordance with an apparatus according to theinvention. In particular there may be provided in accordance with theinvention in a first aspect apparatus for producing a product bycompression of material comprising: a hollow die; an elasticallycompressible lining for the die; means for compressing the lining toprovide in the die a compressed lining of reduced internal size; meansfor compressing material in the interior of the lining when inside thedie, and means for releasing the lining at least partially from the dieto produce an increase in the internal size of the lining to allowremoval of the compressed product from the lining, in which the liningis a continuous lining for the interior of the die. Preferably thelining has, when uncompressed, an external size greater than theinternal size of the die, and the means for compressing the liningcomprises means for forcing the elastically compressible lining into thedie so as to compress the lining.

In accordance with another aspect of the invention, there may beprovided apparatus for producing a product by compression of materialcomprising: a hollow die; an elastically compressible lining for thedie; means for compressing the sleeve to provide in the die a compressedsleeve of reduced internal size; means for compressing material in theinterior of the sleeve when inside the die, and means for releasing thesleeve at least partially from the die to produce an increase in theinternal size of the sleeve to allow removal of the compressed productfrom the sleeve, in which the interior surface of the die and theexterior surface of the sleeve are both tapered and the means forcompressing the sleeve comprises means for forcing the sleeve into thedie independently of the means for compressing the material in theinterior of the sleeve.

There is also provided in accordance with the invention a product formedby compression of material in accordance with the steps of the methodset out, or by use of the apparatus set out.

The invention can provide simple means which have been found to beeffective in overcoming the problems set out hereinbefore enabling highpressing pressure to be applied whilst still being able to remove thepressed product from the die without damage to either the compact or thedie set.

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings in which:

FIGS. 1a, b and c are diagrammatic representations, in cross section, ofknown apparatus for producing a product by compression of powderedmaterial;

FIGS. 2 and 3 are diagrammatic representations, in cross section,showing the ejection of a compressed product from a die, in accordancewith known arrangements;

FIGS. 4 and 4a to 4f are diagrammatic representations in cross sectionof apparatus embodying the invention for producing a product bycompression, and illustrate steps in the method of use of this apparatus

FIG. 5 is a graph showing diagrammatically the relationship between theextent of insertion of a sleeve in a die of the invention, and thechange in inner diameter of the sleeve, in relationship to load appliedto the sleeve;

FIG. 6 is a graph showing the relationship between density andspringback of a compact formed in an embodiment of the invention;

FIG. 7 is a graph showing the relationship between pressing pressureduring formation of a compact, the density of the compact, and thespringback of the compact after release from the die;

FIG. 8 is a graph showing diagrammatically the relationship between theextent of insertion of a sleeve in accordance with an embodiment of theinvention into a die, the load applied to the sleeve, and the change ininner diameter of the sleeve during insertion;

FIG. 9 is a cross-section through a production tooling apparatus forproducing a compressed product, embodying the invention, showing theapparatus at the beginning of a compression cycle;

FIG. 10 is a cross-section of the apparatus of FIG. 9, shown at the endof a compression cycle;

FIGS. 11a and 11b show respectively a plan view and a section alonglines B--B in FIG. 11a of a sleeve suitable for use in the apparatus ofFIGS. 9 and 10, to produces a gearwheel;

FIGS. 12a and 12b show respectively a plan view and section along linesB--B in FIG. 12a of a gearwheel produced by the sleeve of FIGS. 11a and11b.

As has been described in the introduction to the specification, FIGS. 1ato 1c illustrate a known apparatus for producing a product bycompression, comprising a die 12 and upper and lower punches 13 and 14for compressing powdered material 11, to produce a compact 15. FIG. 2illustrates the shear forces which arise during ejection of the compact15, and FIG. 3 illustrates the change of diameter which occurs in thecompact during ejection, in known methods. FIGS. 4 and 4a to 4f arediagrammatic representations in cross section of apparatus embodying theinvention for producing a product by compression, and illustrate stepsin the method of use of this apparatus. In these Figures, componentscorresponding to components shown in previous Figures are indicated bylike reference numerals. As shown in FIG. 4, the modifications to thedie set in accordance with this embodiment of the invention involve theintroduction of a relatively thin, elastically deformable inner sleeve16 to the die 12 as shown in FIG. 4. This sleeve 16 has an externaltaper Te, which is matched by an internal taper Ti in the die bore, andhas an unstressed inner diameter of Ds.

One method of operation is as follows. In the first step, the innersleeve 16 is pressed into the die 12 as shown in FIG. 4a. During thismovement a compressive stress is generated in the sleeve 16 and theinner diameter Ds of the sleeve is reduced by an amount ds which isdependant on the relative movements of the inner sleeve with respect tothe die 12. The further the sleeve is pressed into the die the greaterwill be the value of ds. This movement has to be elastic in nature suchthat when the inner sleeve 16 is subsequently pushed out of the die, theinner diameter recovers to its former value Ds. Next, the bottom punch14 is entered into the die and the powder 11 is placed in the innersleeve 16. The top punch 13 is then inserted into the die, as shown inFIG. 4b. The top punch and bottom punches 13 and 14 are pressed into thedie to compact the powder 11, as shown in FIG. 4c. At this stage theinner diameter of the sleeve 16 is DS-ds and the diameter of thecompressed compact 15 is also Ds-ds. The next step is that the top punch13 is removed, as shown in FIG. 4d. The bottom punch 14, inner sleeve 16and the compact 15 are then all moved upwards together relative to thedie 12, releasing the inner sleeve 12 from the taper of the bore, asshown in FIG. 4e. During this step the inner diameter of the sleeve 16recovers to its original diameter Ds. As the diameter of the innersleeve 16 is increased to this original diameter the compact alsoincreases in diameter due to the `spring-back` effect, that is to saythe relief of the elastic stresses in the compact due to the pressingoperation. The diameter of the compact becomes Ds-ds+dc, where dc is thechange in diameter due to the spring-back effect. Lastly, the compact 15is ejected from the inner sleeve 16 by moving the bottom punch 14relative to the inner sleeve 16, as shown in FIG. 4f.

Two significant points arise. If the value of ds is arranged so that itis equal to, or slightly greater than, dc, then at the last step theinner sleeve 16 will not be in contact with the compact 15, and theejection force required for the last step will be low. Also, it is to benoted that in the movement of the sleeve 16 and compact 15 to partiallyrelease the sleeve and the compact from the die 12 (the movement fromFIG. 4d to FIG. 4e), the internal diameter of the sleeve 16 resumes itsprevious diameter of Ds in a single movement which is uniform throughoutthe height of the sleeve 16. This arises because the sleeve 16 isreleased from the taper of the bore of the die 12 uniformly throughoutits length. The advantage is that the compact 15 is allowed to expand toits final diameter in a uniform movement throughout the length of thecompact. This avoids cracking due to gradual change of diameter as shownin the known arrangement of FIG. 3.

In practice, for any specific pressing pressure the value of dc can beobtained experimentally by pressing compacts in a die of fixed size andmeasuring the diameter of the compact on ejection. The sleeve is thendesigned such that ds is greater than dc. This design be either bycalculation from the known mechanical properties of the sleeve materialsused, or by trial and error. The essential part of the process in theembodiment described is that the internal diameter of the inner sleevehas to decrease elastically before or during the pressing operation, andon removal of the sleeve from the die an elastic recovery of theinternal diameter of the die takes place, preferably slightly greaterthan the elastic recovery of the external diameter of the compact.Although the geometry has been described in terms of a solid cylindricalcomponent, the technique is applicable to other shapes, for examplewashers or hollow cylinders. These may have non-circular externalshapes, such as various gear forms. It is also to be appreciated thatthe technique can be used for the re-repressing of partially sinteredpowder metallurgy compacts, and also fully sintered compacts either toincrease their density or to press them to final, accurate, dimensions.

There will now be described with reference to FIGS. 4 and 4a, and FIGS.5 to 8, a method of calibrating the die and lining shown in FIGS. 4 to4f. In summary, this calibration is achieved by the steps of:

(a) pressing the sleeve into the die and measuring the change in innerdiameter of the sleeve as a function of the change in axial position ofthe sleeve;

(b) for any particular product material, measuring the compressibilityand spring back as a function of the pressing pressure;

(c) for a required pressing density during production of a compressedproduct, determining from the information of step (b) the spring backwhich would occur in a conventional die; and

(d) determining from the data acquired in step (a) the extent ofinsertion of the sleeve that is required to give a value of decrease ofinner diameter of the sleeve which is equal to the expected spring backdetermined in step (c), or falls within a predetermined range ofdeviation from that springback.

Referring to FIG. 4, the reference letter h indicates the height of thesleeve 16 above the top of the die 12, and L indicates the load on thesleeve 16 during insertion of the sleeve into the tapered bore in thedie 12. The first calibration step, step (a), consists of pressing thesleeve 16 into the die 12 under the load L and measuring the change inthe protruding height h and the change in the inner diameter of thesleeve ds which results. The inter-relationship between these measuredparameters, is shown diagrammatically in FIG. 5. In this Figure theabscissa coordinate of the graph shows change in height h. The ordinatecoordinate shows for the broken line the load L, and for the continuousline, the change in inner diameter ds of the lining 16.

The second step of calibration, step (b), is the measurement for anyparticular powder, of the compressibility and springback as a functionof pressing pressure. The springback is the difference between the innerdiameter of the die and the outer diameter of the compact when ejectedfrom the die. The relationship between density and springback is shownschematically in FIG. 6. In this figure the ordinate coordinate showsthe pressing pressure acting on the powder during formation of thecompact. The abscissa coordinate shows in respect of the broken line thedensity of the compact after termination at a given pressing pressureand after ejection from the die. The ordinate coordinate shows inrespect of the continuous line the springback of the compact afterejection from the die.

The third step of calibration, step (c), is the determination, for arequired final pressing density do, the springback dco that would occurin conventional dies such as those illustrated in FIGS. 1a to 3. Therelationship of this springback dco is shown in FIG. 7, in which theabscissa coordinate indicates pressing pressure during formation of thecompact. The ordinate coordinate shows in respect of the broken line thedensity of the compact and shows in respect of the continuous line thespringback dc.

The fourth step, step (d), is to determine the change in height ho thatis required to give a value of ds equal to dco, as illustrated in FIG.8. In FIG. 8 the abscissa coordinate shows change in h. The ordinatecoordinate shows, in respect of the broken line the change in innerdiameter ds, and shows in respect of the continuous line the load Lapplied to force the sleeve into the die. Determination of the change inheight ho that is required to give a value of ds equal to dco,effectively ensures that the elastic recovery of the die diameter onejection is equal to increase in diameter of the compact whenunconstrained.

There will now be described with reference to FIGS. 9 and 10 an exampleof production tooling to put into effect the embodiment of the inventionexplained diagrammatically with reference to FIGS. 4 to 4f. Componentswhich correspond to components in the earlier figures are indicated inFIGS. 9 and 10 by the same reference numerals. A die 12 has an internaltaper along its internal face 17, and a sleeve 16 has a taper on itsexternal face 18, corresponding to the taper of the die 12. The taper isapproximately 2°. In FIG. 9 a lower punch 14 is shown and in FIG. 10 thelower punch 14 and an upper punch 13 are both shown. The finishedproduct, a compact 15, is in this case in the shape of a ring, formed byan internal core 19, centrally placed in the bore of the die 12. Thecore 19 is moveable vertically during compression to accommodate thedownward movement of the upper punch 13, in conventional manner. In theexample shown, the core 19 is conventional, of constant outer diameter,but other embodiments the core 19 may be made to expand elasticallybefore compression, and to contract on release of the compact from thedie, in accordance with the present invention. FIG. 9 shows theapparatus in an initial stage of the filing and compressing cycle, andFIG. 10 shows the apparatus in the final stage when the compact 15 hasbeen fully compressed.

The tooling consists of a die holder 20 into which is located the die12. The die 12 is a multicomponent die, but is assembled so as to be asingle continuous unit. Two low pressure seals 21 and 22 are positionedbetween the die 12 and die holder 20. A radial member 23 engages sleeve16 at the top thereof, in a cooperating circumferential groove 24 in thesleeve 16. The radial member 23 is bolted to a piston 25 which can movevertically relative to the die holder 20 and a outer retaining structure26, which is bolted to the die holder 20. The radial member 23 isactuated by the piston 24 in operation as will be explained hereinafter.The piston 25 can slide in the annular opening provided between the dieholder 20 and the outer retaining structure 26. The piston 25 has alower space 27 into which oil can be pressurised to move the piston 25upwardly, and therefore to push out the sleeve 16 from the die 12. Thelower space 27 is contained by high pressure seals 28, 29 and 30. Anupper space 31 is provided into which oil may also be pressurized in acontrolled cycle, to move the piston 25 downwardly and consequently tomove the sleeve 16 into the die 12. The upper space 31 is contained byhigh pressure seals 28 and 32. The whole assembly is held in a pressbolster by the retaining structure 26. A subsidiary power pack (notshown) delivers high pressure oil to the upper and lower spaces 31 and27 at the correct time during the press cycle. These times are takenfrom a master cam (not shown) on the press, the position of which isconverted into press angle, between 0 and 360° in conventional manner.By way of example, the dimensions of the sleeve may be as follows.

    ______________________________________                                        Length:           103.60 mm                                                   Inner diameter:   44.66 mm                                                    Outer diameter at top:                                                                          55.85 mm                                                    Outer diameter at bottom:                                                                       49.68 mm                                                    Depth of groove 24:                                                                             10.00 mm                                                    ______________________________________                                    

The operation of the apparatus will now be described. Starting from aninitial position shown in FIG. 9 with the top punch 13 removed from thedie 12, the cycle is as follows. The upper space 31 is pressurized topush the sleeve 16 into the die 12. The internal dimensions of thesleeve 16 are consequently reduced, as has been explained hereinbefore.The degree of reduction of internal dimensions of the sleeve can bevaried, by varying the degree of movement of the radial member 23 by thepiston 25. Conveniently the degree of movement of the sleeve into thedie can be determined by placing spacers between the radial member 23and the top of the die 12. In the present case, pressurized oil isadmitted to the upper space 31 until the undersurface of the radialmember 23 rests on the upper surface of the die 12. The powder to formthe compact 15 is then placed in the interior of the lining 16 of thedie 12, in this case with a core 19 protruding upwardly through thepowder. The top punch 13 then enters the die and descends relative tothe lower punch 14 and the sleeve 16. The compact 15 is thus produced bycompression, as shown in FIG. 10. During the entry of the upper punch 13into the die, the core 19 descends to the position shown in FIG. 10.During the compression, the lower punch 14 rises relative to the sleeve16 and the powder 15.

In practice in the embodiment shown, the movements which have beendescribed in relative terms, are not absolute. In known manner in doubleended presses, the lower punch 14 stays stationery in an absoluteposition in the press bolster and the effect of the lower punchcompressing the material is achieved by the entire assembly of dieholder 20 and retaining structure 26, being lowered during the presscycle. Thus the double ended compression is achieved by the lower punch14 remaining stationary the die 12 descending through one measureddistance, and the upper punch 13 descending through twice thepredetermined distance.

After the compression is completed as shown in FIG. 10, the upper space31 is depressurized. The top punch 13 is withdrawn by the normal presscycle. The lower space 27 is pressurized to push upwardly the sleeve 16with the compact 16 still inside it. During the sleeve withdrawal theinternal dimensions of the sleeve revert to their original, largerdimensions, and there is no relative vertical movement between thecompact 15 end the sleeve 16 during this expansion. The bottom punch 14is then used to eject the compact from the sleeve. The lower space 27 isthen finally depressurised.

The materials used for the die 12, the sleeve 16 and the punches 13 and14, are conventional tool steel compositions, conveniently AISI D3/D6.Examples are as follows.

                  TABLE A                                                         ______________________________________                                        Tooling Materials                                                             Composition of Tooling Materials by weight %                                  Material                                                                              C      Cr     Mo  V    Mn  W   Si   Co  Ni  Fe                        ______________________________________                                        AISI D2 1.55   12     0.7 1                         bal                       AISI D3/D6                                                                            2.05   12.5            0.8 1.3 0.3          bal                       AISI M2 0.9    4.1    5   1.9      6.4              bal                       AISI M3/2                                                                             1.28   4.2    5   3.1      6.4              bal                       ______________________________________                                    

FIGS. 11a and 11b show respectively a plan view and a section of asleeve suitable for use in the apparatus of FIGS. 9 and 10, to produce agearwheel shown in FIGS. 12a and 12b. The dimensions of such a componentand sleeve may be as follows. Outer diameter of sleeve at top 102.20 mm;outer diameter of sleeve at bottom 98.00 mm; length of sleeve 60.00 mm;taper of sleeve 2°; outer diameter of gear wheel 93 mm.

EXAMPLES

There will now be described a series of examples of the production ofcompacts of different materials, made by a conventional method and bythe method of the invention. Where a compact is produced by aconventional die, the die is a double ended pressing die such as shownin FIGS. 1a to 3. Where a compact is made in accordance with theinvention, it is made by a double ended pressing apparatus of the kindshown diagrammatically in FIGS. 4 to 4e, and, in a production example,in FIGS. 9 and 10. Where reference is made to the use of hand set dies,this refers to a hand-operated trial set of dies and punches. Wherereference is made to production tooling, this refers to the productiontooling apparatus shown in FIGS. 9 and 10. The materials used in theexamples are as follows.

                                      TABLE B                                     __________________________________________________________________________    Composition of Materials Used for Compacts                                             Composition by Weight %                                              Material C  S  P  Mn Mo Ni Si Cr Cu Fe                                        __________________________________________________________________________    NC100.24 0.02                       bal                                       Atomet 1001                                                                            0.003                      bal                                       Atomet 4601                                                                            0.003                                                                            0.009                                                                            0.012                                                                            0.2                                                                              0.55                                                                             1.8                                                                              0.003                                                                            0.005                                                                            0.02                                                                             bal                                       316L stainless st.                                                                     0.016                                                                            0.009    2.55                                                                             12.9                                                                             0.88                                                                             17.9  bal                                       __________________________________________________________________________

In each table of results in the Examples, the headings of the columnshave the following meanings. Pressing Pressure indicates the pressure intons per square inch applied to the powder to be compressed, by thedouble ended pressing. Density indicates the density of the compact ingrammes per cc, after ejection of the compact from the press. %springback indicates the expansion of the compact after ejection fromthe die, defined as follows: ##EQU1##

The column headed "Die scoring or compact cracking" indicates by an xthose samples where unacceptable difficulties arose from the highDressing pressure used, either by scoring of the internal surface of thedie due to sticking of the compact during ejection, and/or the presenceof cracking in the compact after ejection, due to the partial expansionof the compact as it became partially ejected from the die.

Example 1 (NC100.24)

Cylindrical compacts were made from NC100.24 ferrous powder using aconventional double ended pressing die with different amounts oflubrication, by zinc stearate, giving the following results.

                  TABLE 1                                                         ______________________________________                                        Conventional Pressing                                                         Material Compressed: NC100.24 + 0.8% zinc stearate.                           Pressing                                                                              Density              Die scoring or compact                           Pressure tsi                                                                          g/cc      % Springback                                                                             cracking                                         ______________________________________                                        45.2    7.05      0.281                                                       50.9    7.08      0.307                                                       56.5    7.14      0.346                                                       65.7    7.16      0.316      x                                                78.8    7.21      0.335      x                                                ______________________________________                                    

Table 2: Conventional Pressing

                  TABLE 2                                                         ______________________________________                                        Conventional Pressing                                                         Material Compressed: NC100.24 + 0.6% zinc stearate                            Pressing                                                                              Density              Die scoring or compact                           Pressure tsi                                                                          g/cc      % Springback                                                                             cracking                                         ______________________________________                                        52.5    7.13      0.252                                                       65.7    7.25      0.292                                                       78.8    7.29      0.328      x                                                ______________________________________                                    

Table 3: Conventional Pressing

                  TABLE 3                                                         ______________________________________                                        Conventional Pressing                                                         Material compressed: NC100.24 + 0.4% zinc stearate                            Pressing                                                                              Density              Die scoring or compact                           Pressure tsi                                                                          g/cc      % Springback                                                                             cracking                                         ______________________________________                                        28.3    6.63      0.171                                                       33.9    6.88      0.207                                                       39.6    6.99      0.244                                                       45.2    7.12      0.265      x                                                50.9    7.18      0.289      x                                                65.7    7.32      0.284      x                                                78.8    7.38      0.328      x                                                ______________________________________                                    

Table 4: Conventional Pressing

                  TABLE 4                                                         ______________________________________                                        Conventional Pressing                                                         Material Compressed: NC100.24 + 0.2% zinc stearate.                           Pressing                                                                              Density              Die scoring or compact                           Pressure tsi                                                                          g/cc      % Springback                                                                             cracking                                         ______________________________________                                        11.3    5.49      0.102                                                       16.9    6.03      0.118                                                       22.6    6.34      0.131                                                       28.3    6.63      0.173      x                                                33.9    6.84      0.184      x                                                ______________________________________                                    

A series of compacts was then produced by means of an elasticallycompressible lining in a method embodying the invention. Compacts wereproduced using a hand set as shown in FIGS. 4 to 4f, and having thefollowing parameters.

                  TABLE C                                                         ______________________________________                                        Handset. Hand Operated die set                                                Applied                   Reduction in                                        Sleeve Load   Diameter Ds Diameter ds                                         tonnes        mm          mm        % ER                                      ______________________________________                                        0             32.157      0         0                                         5             32.0895     0.0675    0.21                                      7.5           32.074      0.078     0.243                                     10            32.06       0.097     0.302                                     Ejection load on sleeve 5 t.                                                  ______________________________________                                    

Applied sleeve load means the load in tons applied to the top of thesleeve (for example as shown in FIGS. 4 and 4a) to force the sleeve intothe tapered die. Diameter Ds means the diameter of the interior of thesleeve which diminishes as the sleeve is forced into the conical die,measured in millimeters. Reduction in diameter, ds, means the reductionin the internal diameter of the sleeve produced by application of theload shown. % ER means the elastic recovery of the sleeve after releasefrom the die measured as a % of the increase in internal diameter of thesleeve upon release, defined as follows: ##EQU2##

Cylindrical compacts were made from NC100.24 ferrous powder using adouble ended pressing die embodying the invention, as shown in FIGS. 4to 4f, with different amounts of lubrication by zinc stearate, with thefollowing results.

                  TABLE 5                                                         ______________________________________                                        Pressing by an Embodiment of the Invention                                    Material compressed: NC100.24. Hand Operated Die Set.                         Sleeve fully inserted. No die scoring or compact cracking found.                                              Pressing                                                                            Eject.                                                                              Dens-                                      ds                     Pressure                                                                            pressure                                                                            ity                               Lubrication                                                                            (mm)   % ER   Springback %                                                                           tsi   tons  g/cc                              ______________________________________                                        0.8% zinc                                                                              0.097  0.302  0.28 at 50 tsi                                                                         48    0    7.13                               stearate                                                                      0.8% zinc                                                                              0.097  0.302  0.295 at 55 tsi                                                                        56    >5 t 7.24                               stearate                                                                      0.8% zinc                                                                              0.097  0.302  0.325 at 65 tsi                                                                        64    >5 t 7.29                               stearate                                                                      0.8% zinc                                                                              0.097  0.302  0.34 at 70 tsi                                                                         72    >5 t 7.31                               stearate                                                                      0.8% zinc                                                                              0.097  0.302  0.356 at 80 tsi                                                                        80    >5 t 7.39                               stearate                                                                      die wall 0.097  0.302  0.120 at 40 tsi                                                                        48    0    7.19                               lubrication                                                                   die wall 0.097  0.302  0.120 at 40 tsi                                                                        56    0    7.33                               lubrication                                                                   die wall 0.097  0.302  0.120 at 40 tsi                                                                        64    0    7.43                               lubrication                                                                   die wall 0.097  0.302  0.120 at 40 tsi                                                                        72    0    7.4                                lubrication                                                                   die wall 0.097  0.302  0.259 at 80 tsi                                                                        80    0    7.54                               lubrication                                                                   ______________________________________                                    

A series of compacts was then produced using the production tooling asshown in FIGS. 9 and 10, and having the following parameters.

                  TABLE D                                                         ______________________________________                                        Production Tooling                                                            Sleeve Internal diameter                                                      Initial diameter (mm) 44.665                                                  Elastically constrained diameter (mm)                                                               44.504                                                  % Elastic recovery possible                                                                         0.34                                                    ______________________________________                                    

Cylindrical compacts were made from NC100.24 ferrous powder using adouble ended pressing die embodying the invention, as shown in FIGS. 9and 10, with different amounts of lubrication by zinc stearate and withwall lubrication, with the following results.

                  TABLE 6                                                         ______________________________________                                        Pressing by an Embodiment of the Invention                                    Material compressed: NC100.24. Production Tooling.                            Sleeve fully inserted. No die scoring or cracking of compacts found.                    ds            Springback                                                                             Pressing                                                                              Density                              Lubrication                                                                             (mm)   % ER   %        Pressure tsi                                                                          g/cc                                 ______________________________________                                        0.8% zinc stearate                                                                      0.161  0.34   0.34 at 70 tsi                                                                         70      7.09                                 0.4% zinc stearate                                                                      0.161  0.34   0.32 at 65 tsi                                                                         65      7.31                                 0.4% zinc stearate                                                                      0.161  0.34   0.33 at 75 tsi                                                                         75      7.35                                 0.4% zinc stearate                                                                      0.161  0.34   0.33 at 75 tsi                                                                         75      7.4                                  + die wall lubric.                                                            die wall lubric.                                                                        0.161  0.34            75      7.5                                  ______________________________________                                    

The results in the tables, Table 5 and Table 6, are comparable withresults in Tables 1, 2, 3 and 4. Note that, in Table 1, 2, and 3, as theamount of lubricant in the powder decreases the compacts become more andmore difficult to eject from the conventional die without damage. Thesafe pressing pressure drops from about 55 tsi with 0.8% zinc stearateto about 25 tsi with 0.2% zinc stearate added as lubricant. Table 5shows that all compacts in the elastic die handsets were ejected withoutdamage and with low ejection forces. It can also be seen in Table 5,that as the expected springback (% SB) of the compressed powder compactincreases, (figures taken from data in Table 1) the ejection force onlybecomes positive when its value exceeds the elastic recovery (% ER) ofthe sleeve. With die wall lubrication in Table 5, all compacts wereejected with zero ejection force as the expected % springback even at 80tsi (0.259%) was less than the elastic recovery of the sleeve (0.302%).

Table 6 illustrates that the production tooling, designed to give anelastic recovery (0.34%), approximately equal to the springback expectedwith NC100.24 at 80 tsi using die wall lubrication (0.34%), producedsound compacts of high density with practically zero ejection force.

Example 2 (316L Stainless Steel)

A similar series of sets of compacts was then produced using 316Lstainless steel, by conventional means, and by embodiments of thepresent invention, with the following results.

                  TABLE 7                                                         ______________________________________                                        Conventional Pressing                                                         Material Compressed:                                                          316L Stainless steel + 1% lithium stearate                                    Pressing                                                                              Density              Die scoring or compact                           Pressure tsi                                                                          g/cc      % Springback                                                                             cracking                                         ______________________________________                                        39.6    6.6       0.254                                                       45.2    6.73      0.283                                                       50.9    6.83      0.294                                                       56.5    6.94      0.323      x                                                65.7    7         0.324      x                                                78.8    7.1       0.358      x                                                ______________________________________                                    

Table 8: Conventional Pressing

                  TABLE 8                                                         ______________________________________                                        Conventional Pressing                                                         Material Compressed:                                                          316L stainless steel + 0.6% lithium stearate                                  Pressing                                                                              Density              Die scoring or compact                           Pressure tsi                                                                          g/cc      % Springback                                                                             cracking                                         ______________________________________                                        33.9    6.43      0.257                                                       39.6    6.57      0.275                                                       45.2    6.7       0.294                                                       50.9    6.83      0.312      x                                                56.5    6.93      0.338      x                                                65.7    7.01      0.338      x                                                78.8    7.15      0.344      x                                                ______________________________________                                    

                  TABLE 9                                                         ______________________________________                                        Conventional Pressing                                                         Material Compressed:                                                          316L stainless steel + 0.4% lithium stearate                                  Pressing                                                                              Density              Die scoring or compact                           Pressure tsi                                                                          g/cc      % Springback                                                                             cracking                                         ______________________________________                                        16.9    5.7       0.215                                                       22.6    6.01      0.244                                                       28.3    6.24      0.257                                                       33.9    6.54      0.265      x                                                39.6    6.58      0.275      x                                                45.2    6.73      0.299      x                                                50.9    6.86      0.331      x                                                56.6    6.92      0.341      x                                                65.7    7.07      0.312      x                                                78.8    7.17      0.34       x                                                ______________________________________                                    

                  TABLE 10                                                        ______________________________________                                        Pressing by an Embodiment of the Invention                                    Material compressed: 316 Stainless Steel. Hand Operated Die Set.              Sleeve fully inserted. No die scoring of compact cracking found.                                              Pressing                                                                            Eject.                                           ds                     Pressure                                                                            press.                                                                             Density                            Lubrication                                                                            (mm)   % ER   Springback %                                                                           tsi   tons g/cc                               ______________________________________                                        1% lithium                                                                             0.097  0.302  0.29 at 50 tsi                                                                         48    0    6.77                               stearate                                                                      1% lithium                                                                             0.097  0.302  0.31 at 55 tsi                                                                         56    >5 t 6.94                               stearate                                                                      1% lithium                                                                             0.097  0.302  0.33 at 65 tsi                                                                         64    >5 t 7.01                               stearate                                                                      1% lithium                                                                             0.097  0.302  0.34 at 70 tsi                                                                         72    >5 t 7.07                               stearate                                                                      1% lithium                                                                             0.097  0.302  0.328 at 80 tsi                                                                        80    >5 t 7.12                               stearate                                                                      0.4% zinc                                                                              0.097  0.302  0.31 at 50 tsi                                                                         48    0    6.75                               stearate                                                                      0.4% zinc                                                                              0.097  0.302  0.325 at 55 tsi                                                                        56    0    6.88                               stearate                                                                      0.4% zinc                                                                              0.097  0.302  0.34 at 65 tsi                                                                         64    >5 t 7.05                               stearate                                                                      0.4% zinc                                                                              0.097  0.302  0.345 at 70 tsi                                                                        72    >5 t 7.09                               stearate                                                                      0.4% zinc                                                                              0.097  0.302  0.355 at 80 tsi                                                                        80    >5 t 7.21                               stearate                                                                      die wall 0.097  0.302  0.20 at 50 tsi                                                                         48    0    6.61                               lubrication                                                                   die wall 0.097  0.302  0.22 at 55 tsi                                                                         56    0    6.83                               lubrication                                                                   die wall 0.097  0.302  0.275 at 65 tsi                                                                        64    0    6.94                               lubrication                                                                   die wall 0.097  0.302  0.29 at 70 tsi                                                                         72    >5 t 7.09                               lubrication                                                                   die wall 0.097  0.302  0.34 at 80 tsi                                                                         80    >5 t 7.23                               lubrication                                                                   ______________________________________                                    

                  TABLE 11                                                        ______________________________________                                        Pressing by an Embodiment of the Invention                                    Material compressed: 316L Stainless Steel. Production Tooling.                Sleeve fully inserted. No die scoring or cracking of compacts found.                                           Pressing                                                                              Density                              Lubrication                                                                           ds (mm)  % ER   Springback %                                                                           Pressure tsi                                                                          g/cc                                 ______________________________________                                        1% lithium                                                                            0.161    0.302  0.34 at 70 tsi                                                                         70      7.09                                 stearate                                                                      ______________________________________                                    

The results in these tables, Table 10 and Table 11, are comparable withresults in Tables 7, 8 and 9. Note that, in Tables 7, 8 and 9, as theamount of lubricant in the powder decreases the compacts become more andmore difficult to eject from the conventional die without damage. Thesafe pressing pressure drops from about 50 tsi with 1.0% lithiumstearate to about 30 tsi with 0.4% lithium stearate added as lubricant.Table 10 shows that all compacts in the elastic die handsets wereejected without damage and with low ejection forces. It can also be seenin Table 10, that as the expected springback (% SB) of the compressedpowder compact increases, (figures taken from data in Tables 7 and 9)the ejection force only becomes positive when its value exceeds theelastic recovery (% ER) of the sleeve. Note, for example, that theejection force only becomes measurable at 55 tsi using 1% lithiumstearate, at 65 tsi using 0.4% lithium stearate, and at 70 tsi usingonly die wall lubrication. In both cases these pressures are those atwhich the expected springback of the compressed material becomes equalto or exceeds the elastic recovery of the sleeve. Even with die walllubrication in Table 10, all compacts were ejected with zero or lowejection force, as the expected % springback at 70 tsi (0.29%) was equalto the elastic recovery of the sleeve (0.302%).

Table 11 illustrates that the production tooling, designed to give anelastic recovery (0.34%), approximately equal to the springback expectedwith 316L stainless steel at 70 tsi using die wall lubrication (0.34%),produced sound compacts of high density with practically zero ejectionforce.

Example 3 (ATOMET 1001 AND ATOMET 4601)

Table 12 illustrates results with two further iron-based powders, Atomet1001, a pure iron powder, and Atomet 4601 an alloy powder withcompositions as in Table A. In industrial practice it is usuallynecessary to add graphite to ferrous powder mixes for metallurgicalreasons. Springback at various pressing pressures was determined aspreviously described and this data (not included here) is used toexplain the results in Table 12. The results show that even with anaddition of graphite the compacts were all produced without damage atzero or low ejection force. Only when the % springback was equal to orexceeded the elastic recovery (% ER) of the sleeve did the ejectionforce become noticeable. The high densities attainable, up to 7.65 g/ccwithout cracking could not be obtained with conventional tooling.

As stated previously the results also show that when the expectedspringback of the compacted material becomes equal to, or greater thanthe elastic recovery of the sleeve the ejection force become positive,but still small enough to allow compacts to be removed rom the toolswithout damage.

                  TABLE 12                                                        ______________________________________                                        Pressing by an Embodiment of the Invention                                    Material compressed: Various. Hand Operated Die Set.                          Sleeve fully inserted. No die scoring of compact cracking found.                                              Pressing                                                                            Eject.                                           ds                     Pressure                                                                            press.                                                                             Density                            Lubrication                                                                            (mm)   % ER   Springback %                                                                           tsi   t    g/cc                               ______________________________________                                        Atomet 1001                                                                            0.097  0.302           48    0    7.37                               Atomet 1001                                                                            0.097  0.302           56    0    7.48                               Atomet 1001                                                                            0.097  0.302           64    0    7.57                               Atomet 1001                                                                            0.097  0.302           72    0    7.63                               Atomet 1001                                                                            0.097  0.302  0.34 at 80 tsi                                                                         80    0    7.65                               Atomet 1001 +                                                                          0.097  0.302  0.284 at 80 tsi                                                                        80    >5 t 7.59                               0.5% graphite                                                                 Atomet 4601                                                                            0.097  0.302           48    0    7.14                               Atomet 4601                                                                            0.097  0.302           56    0    7.3                                Atomet 4601                                                                            0.097  0.302           64    0    7.41                               Atomet 4601                                                                            0.097  0.302           72    0    7.48                               Atomet 4601                                                                            0.097  0.302  0.284 at 80 tsi                                                                        80    0    7.55                               Atomet 4601 +                                                                          0.097  0.302  0.21 at 50 tsi                                                                         48    0    7.1                                0.5% graphite                                                                 Atomet 4601 +                                                                          0.097  0.302  0.23 at 55 tsi                                                                         56    0    7.32                               0.5% graphite                                                                 Atomet 4601 +                                                                          0.097  0.302  0.27 at 66 tsi                                                                         64    0    7.36                               0.5% graphite                                                                 Atomet 4601 +                                                                          0.097  0.302  0.315 at 75 tsi                                                                        75    >5 t 7.43                               0.5% graphite                                                                 Atomet 4601 +                                                                          0.097  0.302  0.318 at 80 tsi                                                                        80    >5 t 7.5                                0.5% graphite                                                                 ______________________________________                                    

The embodiments described above related to sleeves that form the outsideshape of the component. Centrally placed core rods, and off-centre corerods have to be dealt with in a different mechanical arrangement butstill using the elastic recovery technique. In the case of core rods theexternal dimensions of the core rod have to be made larger beforecompaction. After compaction the original dimensions then need to berecovered, that is the external dimensions decrease. This makes itpossible for the ore rod to be withdrawn from the component with zero,or very much reduced force. This not only prevents damage to thecomponent, but also to the core rod itself. Expansion of the core rod iseffected by having a sleeve on the outside of the core rod with a taperon the insider surface of the sleeve. When the sleeve is pulled over thecore rod, which has a matching taper, or when the core rod is driveninto this external sleeve, the external dimensions of the sleeve areincreased in the same manner that the internal dimensions of the diesleeve decrease when the sleeve is pulled into the die. Aftercompaction, the sleeve is pushed off the core rod, or the core rod iswithdrawn from the sleeve allowing it to elastically recover to itsoriginal smaller external dimensions. The compact is then withdrawn fromthe die and the core rods removed with zero or low force.

I claim:
 1. A method of producing a product by compression of material,comprising the steps of:providing in a hollow die a compressed liningwhich is elastically compressed so as to reduce the internal size of thelining relative to the internal size before compression, compressingproduct material in the lining to produce a compressed product,releasing the lining at least partially from the die to produce anincrease in the internal size of the lining, and removing the compressedproduct from the lining, in which the lining is continuous around theinterior of the die, and the method includes compressing the lining by asmooth, continuous, elastic deformation of the bulk material of thelining, so as to reduce the internal size of the lining whilemaintaining the accuracy of the internal shape of the lining.
 2. Amethod according to claim 1 including the step of compressing the liningbefore the step of compressing the product material to produce thecompressed product.
 3. A method according to claim 2 including the stepof producing an adjustable, selectable, compression of the lining,whereby the increase in internal size of the lining on release of thelining from the die can be selected in relation to the expected increasein external size of the product on release from the die.
 4. A methodaccording to claim 2 including compressing the lining to an extent suchthat the increase in internal size of the lining on release of thelining from the die is in the range + or -10% of the increase inexternal size of the product on release from the die.
 5. A methodaccording to claim 2 including compressing the lining to an extent suchthat the increase in internal size of the lining during the release fromthe die is substantially equal to the expansion of the product afterrelease from the die.
 6. A method according to claim 1 in which thelining is a sleeve and the method includes inserting the sleeve into thedie along the direction of a common axis of the sleeve and the die, andin which the interior of the die and the exterior surface of the sleeveare both tapered in the direction of the common axis so that insertionof the sleeve produces compression of the sleeve by the die.
 7. A methodaccording to claim 6 in which the angle of taper of the die and thesleeve is in the range 1 to 5°.
 8. A method according to claim 7 inwhich the angle of taper of the die is about 2°.
 9. A method accordingto claim 6 in which the hollow die is provided by an aperture in a dieand the method includes compressing the material by moving upper andlower punches into the aperture in the die in the interior of thelining.
 10. A method according to claim 6 including calibrating the dieand lining by the steps of:(a) pressing the sleeve into the die andmeasuring the change in inner diameter of the sleeve as a function ofthe change in axial position of the sleeve; (b) for any particularproduct material, measuring the compressibility and spring back as afunction of the pressing pressure; (c) for a required pressing densityduring production of a compressed product, determining from theinformation of step (b) the spring back which would occur in aconventional die; and (d) determining from the data acquired in theextent of insertion of the sleeve that is required to give a value ofdecrease of inner diameter of the sleeve which is equal to the expectedspring back determined in step (c), or falls within a predeterminedrange of deviation from that springback.
 11. A method according to claim1 including forming an opening in the compressed product by a coreprovided in the hollow die, including the steps of:expanding elasticallythe external size of the core and compressing the product material inthe die around the expanded core; after the compression of the productmaterial, reducing the external size of the core; and removing the corefrom the compressed product.
 12. Apparatus for producing a product bycompression of material comprising:a hollow die; an elasticallycompressible lining for the die; means for compressing the lining toprovide in the die a compressed lining of reduced internal size; meansfor compressing material in the interior of the lining when inside thedie, and means for releasing the lining at least partially from the dieto produce an increase in the internal size of the lining to allowremoval of the compressed product from the lining, in which the liningis a continuous lining for the interior of the die, and the lining haswhen uncompressed, an external size greater than the internal size ofthe die, the means for compressing the lining comprising means forforcing the elastically compressible lining into the die to compress thelining by a smooth, continuous, elastic deformation of the bulk materialof the lining, so as to reduce the internal size of the lining whilemaintaining the accuracy of the internal shape of the lining.
 13. Amethod of producing a product by compression of material, comprising thesteps of:providing in a hollow die a core to form a required opening inthe final product; compressing product material in the die around thecore to produce a compressed product; and after the compression of theproduct material, removing the compressed product from the die and fromthe core; in which the method includes expanding elastically theexternal size of the core and compressing the product material in thedie around the expanded core, and, after the compression of the productmaterial, reducing the external size of the core to assist removal ofthe compressed product from the core, the core being continuous aroundits external surface, and the exterior of the core being expanded by asmooth continuous elastic deformation so as to increase the externalsize of the core while maintaining the accuracy of the external shape ofthe core.
 14. A method of producing a product by compression ofmaterial, comprising the steps of:providing in a hollow die a compressedsleeve which is elastically compressed so as to reduce the internal sizeof the sleeve relative to the internal size before compression,inserting into the compressed sleeve a material to be compressed,compressing the material in the sleeve to produce a compressed product,releasing the sleeve at least partially from the die to produce anincrease in the internal size of the sleeve, and removing the compressedproduct from the sleeve, in which the interior surface of the die andthe exterior surface of the sleeve are both tapered, and the methodincludes the step of inserting the tapered sleeve into the tapered dieand compressing the sleeve by the effect of the tapered surfaces beforethe compression of the product material in the sleeve, to produce aselectable compression of the lining depending upon the extent ofinsertion of the sleeve into the die, whereby the increase in internalsize of the lining on release of the lining from the die can be selectedin relation to the expected increase in external size of the product onrelease from the die.
 15. Apparatus for producing a product bycompression of material comprising:a hollow die; an elasticallycompressible lining for the die; means for compressing the sleeve toprovide in the die a compressed sleeve of reduced internal size; meansfor compressing material in the interior of the sleeve when inside thedie, and means for releasing the sleeve at least partially from the dieto produce an increase in the internal size of the sleeve to allowremoval of the compressed product from the sleeve; in which the interiorsurface of the die and the exterior surface of the sleeve are bothtapered, and the means for compressing the sleeve comprises means forforcing the sleeve into the die independently of the means forcompressing the product material in the interior of the sleeve, toproduce a selectable compression of the lining depending upon the extentof insertion of the sleeve into the die, whereby the increase ininternal size of the lining on release of the lining from the die can beselected in relation to the expected increase in external size of theproduct on release from the die.